The invention relates to a device for increasing security with respect to transmission faults in an information-transmission system, particularly for the railroad field, in which an interrogation device and a passive response device are movable relative to one another, and the interrogation device transmits an interrogation signal, which can be periodically swept through a predetermined frequency band, preferably in the microwave range, and which during a read operation, receives during several periods from the passive response device, a frequency-selectively reflected interrogation signal, in accordance with a k-digit information item to be transmitted, which signal forms the response signal.
This type of microwave transmission system for sequential or successive information transmission between a passive response device and an active inquiry device is described, for example, in the publication "aci-report 8" of the Siemens Company. The system can be utilized for transmission of car or vehicle data to a stationary line or track installation, or line or track data to a car or vehicle in dependence upon the disposition of the interrogation and the cooperable passive devices.
As a result of the high reading speed of the interrogation device, each response device repeatedly transmits its information, in coded form, a number of times. Consequently, it is possible by means of suitable code and identify checking devices to select that information item of those transmitted during a read operation, with the fewest errors. Selection thereof, can be accomplished with a device such as illustrated in German Pat. No. 2,101,105, as a result of which a substantial increase in the transmission security of this system can be achieved. Such known checking device has the characteristic, in a read operation, of evaluating as "correct" information which it has recognized as identical to prior information, with such information then being through-connected for transmission by the response device to further processing devices, independently of whether additional following code correct and identical pieces of information are determined during such a read operation.
It will be appreciated that during the reflection from a response device, the reflected signal of the interrogation device, representing the response signal for the inquiry device, will appear as a useful signal in the receiving branch of the interrogation device, and such useful signal will rise above a noise level occasioned, for example, by interfering signals, such as interferring reflections, with the response signal increasing to a maximum value and then diminishing. Falsification in such a response signal which cannot be recognized by the code and identity checking device most probably occurs in a transition range in which the interference signal level and the useful signal level are of about equal magnitude. However, if this type of falsified information has once been evaluated as "correct," in a device such as that illustrated in German Pat. No. 2,101,105, such information is undesirably through-connected independently of whether true and correct information occurs during the remainder of the read operation involved.